This invention relates to a telephone line d.c. loop current regulator.
When private switching systems are connected to a public telephone exchange it is necessary for them to regulate the current on the line between the private switching system and the public exchange. The d.c. current regulation has to be performed in the presence of ac signals such as speech. Electronic circuits which terminate analogue telephone lines should behave like standard telephone terminals as seen from the public exchange. One of the requirements is to have a dc current flow to indicate that the terminal is off hook, that is that the terminal wishes to make a telephone call. The dc current should follow voltage/current characteristics which are specified by the network provider. Voice band ac signals should not be affected by the d.c. current regulating circuit. The terminating equipment needs an indication that a current above a threshold level is present. Loop current regulating and detection circuits are usually implemented using inductors in the form of coils to provide the required high ac impedance. At voice band frequencies these coils are relatively large and expensive.